After having done some research and examined a body of literature I come to the conclusion that the most detailed method to simulate the open-loop gain of op-amps should follow the industry-accepted NULL method for operational amplifier testing used in mass production of the devices. Is this method the best way, depends on what criteria you use to define "the best way": availability, implementation difficulty, reliability etc. But what is certain, it is the most detailed method. This method permits to measure not only open-loop gain, but all the other parameters also.
So, NULL method: "a method of measurement in which an unknown quantity (as of electric current) is compared (as in a Wheatstone bridge) with a known quantity of the same kind and found equal by zero response of the detector" (Merriam-Webster dictionary, definition).
This method is recommended as a measurement technique by op-amp datasheets of many manufacturers. The method described in the accepted answer can be interpreted, with some inventiveness, as a null method. For the successful implementation of the NULL method when doing measurements and simulations with op-amp circuits, I recommend you to study the circumstantial account by authors with the Gunma University, Japan and ROHM Co., "Simulation Evaluation of Null Method for Operational Amplifier Testing".
The source being respected, I add, for readers' convenience, some exerpts and drawings from the article.
The operational amplifier has differential inputs with high impedance, a single-ended output with low impedance, and an extremely high gain. Accurate performance measurement is demanded in high
precision analog circuits. However, there is a problem that the high open loop gain prevents accurate performance measurement such as the minute voltage error generation at the amplifier input due to the
influence of peripheral circuits / environments (noise, thermal electromotive force by Seebeck effect, GND return current). Therefore, we have confirmed the operation of the NULL method circuit 1 using SPICE simulations, where the amplifier under test itself measures by using the servo loop to force the amplifier negative input voltage to zero potentiall, and discuss the appropriate selection of capacitor values in the NULL circuit method. In addition, we notice that we describe experiments on this NULL method circuit in , and also the DC-AC conversion technique that realizes a high accuracy test of a minute offset voltage in a short time using multi-channel measurement in .
2. Basic Operational Amplifier Measurement Circuit
The operational amplifier measurement circuit using the NULL method is shown in Fig. 1. The auxiliary operational amplifier is used as an integrator to form a stable loop with extremely high DC open loop gain.
4.3 Open Loop Gain (AOL)
Table 2 shows open loop gain characteristics simulation results for the circuit in Fig. 8, where a
square wave of 1Vp-p and 1Hz is provided to the negative terminal side of the operational amplifier
used as an integrator for the load resistance RL of 2kΩ, 10kΩ and 100kΩ.